The invention relates to a heating and cooling system for air, for example for maintaining a desired air temperature in a building using fresh air or re-circulating room air or a mixture thereof, by using an air-conditioning system which can be "reversed" to function as a heat pump.
It is known that a conventional refrigeration system using a vapour compression refrigerator can be reversed to function as a heat pump. A conventional refrigeration plant cools a space by extracting heat therefrom by evaporation of a refrigerant or working fluid, and rejects the heat outside by condensation. On the other hand, a heat pump utilizes the heat rejected by condensation of the refrigerant to heat a space, and extracts heat from outside by evaporation of the refrigerant.
A refrigeration or heat pump plant is said to be in thermal balance when rates of evaporation and condensation are equal. This requires matching the capacity of the evaporator and condenser as closely as possible, and this matching can present problems. Matching or balancing the capacity of a vapour compression system is particularly difficult if that system is to be reversed to be used as a heat pump as well as an air conditioner. Air conditioners are generally matched to 90 per cent to 95 per cent of the maximum cooling load so as not to run continuously for the majority of the cooling season because they are over-sized when the weather is moderate. An over-sized air conditioner tends to cycle off, that is to switch off, having achieved the space temperature requirements before sufficient de-humidification is accomplished. This results in cool but muggy air within the air conditioned space because the evaporator generally operates below the dew point temperature of the air being treated.
In air-conditioning systems, most of the matching problems arise from the wide variations that can be experienced by the condenser when it is located in ambient air outside the building. Many methods of bringing into balance the rates of evaporation and condensation are available for air-conditioners as will be described. On the other hand, while heat pumps are well known for their relatively high cycle efficiency, problems have arisen in matching the heat rejection capacity of the condenser with the heat absorbing capacity of the evaporator. If the range of outside air temperature passing through the condenser is relatively narrow, matching of the evaporator and condenser capacities is relatively easy and the heat pump can function efficiently. However, if there is a relatively wide range of temperature differences across the condenser of the heat pump, matching of the condenser and evaporator capacities while maintaining high efficiency and high discharge temperature becomes very difficult.
Matching of the capacities of the evaporator and condenser in a refrigerating unit has been accomplished by many different approaches. For a proper balance, refrigerant pressure within the system must be controlled accurately, and this can be done by controlling the rates of condensation and evaporation, using many different methods. One means of approaching balance for common, commercially produced air-conditioners when used in a temperate climate where less condenser capacity is required is to partially flood the condenser with liquid refrigerant, thus effectively reducing condenser area to compensate for cooler ambient air. This requires using larger amounts of refrigerant than would otherwise be required. On the other hand, when attempting to match capacities of the condenser and evaporator in a heat pump system when temperature of air entering the condenser varies, as in fresh air heating, many other difficulties can arise. Typical problems encountered include attempting to maintain design condensing pressure for all outside temperatures with a constant volume make-up air passing through the condenser coil. If the heat pump system has only a single compressor, unloaders using a bypass regulator are required to short circuit refrigerant, thus reducing heat capacity demands but this approach reduces efficiency when attempting to maintain adequate pressure in the condenser.
One factor in compressor matching relates to the density of the vapour to be compressed. Any increase in specific density of the vapour to be compressed requires a corresponding increase in compressor power which is not easily attainable with most single speed compressors. While some compressors are two-speed types, such types require separate windings for each speed, which increases in control complexity, and thus two speed compressors are not common. Running a compressor at low speed to reduce compressor output often increases refrigerant density, which requires a correspondingly higher power input per stroke. Dynamic balancing becomes more difficult at low speed, and oil entrainment for compressor lubrication can suffer at low speed. While in general improved lubrication takes place with high speed compressor operation, adequate return of the oil to the compressor is necessary. Varying operating speed of a compressor can result in poor lubrication and correspondingly aggravate wear of the compressor.
When a heat pump or air conditioner cannot reject all the heat gathered, compressor discharge pressure (i.e. condenser inlet pressure) increases as the vapour occupies a higher specific volume in the condenser than as a condensed liquid. If pressure limiting controls are present, the compressor will shut off until the discharge pressure drops below some nominal reset value. Automatic reset can cause short-cycling of the compressor which pumps oil from the compressor and could result in premature failure due to insufficient lubrication. Alternatively, manual reset of the compressor can be used but this is inconvenient, but it can overcome the short-cycling problems described above.
Constant volume air-conditioners using make-up air have to accommodate wide variations of ambient temperature and still provide a desired cooled space temperature. Clearly, as the ambient temperature rises, more heat must be removed from the air entering the evaporator than with lower ambient temperatures. In order to accommodate these outside air temperature variations in an air conditioner, air flow control means are provided to regulate flow of outside air through the condenser coil of the air-conditioner. The higher the ambient temperature, the more air must pass through the condenser coil and vice versa. Usually air flow control means, termed face dampers, are fitted to control air flow through the main condenser coil. For relatively cool ambient temperatures, where not much heat is removed from incoming air, the face dampers will be closed so that little of the outside air will pass through the condenser. As ambient or outside temperature rises, the face dampers will be opened, to ensure that more outside air will be passed through the condenser.
Many prior art heat pumps have provisions of introducing a limited amount of fresh air into the building stream, and separate exhaust fans expel a similar quantity of exhaust air. As the public's awareness of insufficient fresh air supply to buildings increases, new demands are placed upon heat pumps, air conditioners and other equipment to use greater volumes of fresh air. It is well known that exhausted air contains heat, and while attempts have been made to collect heat from the exhausted air using conventional heat exchangers, such devices are costly, power consuming and relatively inefficient for heat transfer to incoming air.
In all air-conditioning systems known to the inventor, the condenser used to reject heat is located in ambient air, and, to improve heat transfer, the condenser can be cooled by many different methods. The efficiency of heat transfer from the hot refrigerant in the condenser to ambient air is dependent, among other things, on temperature difference between the hot refrigerant and the ambient air. Various means, such as sprayed water cooling for direct evaporation on the coil have been used to assist in condensing the refrigerant.